Nearly 170 tunnels have been found nationwide since 1990, most along the Arizona and California border with Mexico. The job of searching these networks can be dangerous, so the U.S. Border Patrol is unveiling its latest technology in the underground war — a wireless, camera-equipped robot that can do the job in a fraction of the time.

HOW ARE THEY BUILT?

Miners and other laborers hired by the cartels use hoes, jackhammers, shovels and picks to gouge out soil and load the dirt into buckets that are brought back out of the tunnel's starting point in Mexico. The tools that miners use are old-fashioned and can be bought at home improvement stores. Miners, for instance, must use compasses because GPS devices don't work underground.

Smugglers have dug dozens of crude tunnels in Nogales, Ariz., that begin in Mexico and tie into the Arizona city's storm drainage system.

For sophisticated tunnels, such as those found near San Diego that have ventilation and lighting systems and stretch the length of several football fields, cartels will hire engineers and miners to build the tunnels. A cartel will have a financier or a cell that reports to the cartel bosses and runs the construction.

U.S. border officials estimate that the more sophisticated tunnels probably cost between $2 million to $3 million to build.

HOW TUNNELS ARE USED?

Experts say the sophisticated tunnels are used for only drug smuggling, because the people who built it earn big dollars from moving drugs underground and don't want immigrants who are getting smuggled into the United States to spread word about their investment.

A mixture of drugs are brought in through the tunnels, but marijuana — which is big and bulky and therefore difficult to move — is the most prevalent drug transported through the tunnels.

Immigrant smugglers use "gopher hole" tunnels made up of huge PVC pipes that are buried underground and span the border, providing enough space to where a person can barely squeeze through.

The storm-drain tunnels in places like Nogales are used for both immigrant and drug smuggling.

WHY TARGET TUNNELS?

Officials say tons of drugs can be moved through a sophisticated drug tunnel in one day, and until the underground passageway is discovered, the drugs can continue moving unimpeded through the pipeline.

Border Patrol agents are using new robot technology to detect any tunnels that could be used by illegal border-crossers.